The rise of microelectromechanical systems (MEMS) technology has enabled the development of acoustic transducers such as microphones using silicon-wafer deposition techniques. Microphones fabricated this way are commonly referred to as MEMS microphones and can be made in various forms such as capacitive microphones or piezoelectric microphones using a material such as PZT, ZnO, PVDF, PMN-PT, or AlN. MEMS capacitive microphones and electret condenser microphones (ECMs) are used in consumer electronics and have an advantage over typical piezoelectric MEMS microphones in that they have greater sensitivity and lower noise floors. However, each of these more ubiquitous technologies has its own disadvantages. For standard ECMs, they typically cannot be mounted to a printed circuit board using the typical lead-free solder processing commonly used on all other microchips attached to the board. MEMS capacitive microphones, which are often used in cell phones, are relatively expensive due at least in part to the use of an application-specific integrated circuit (ASIC) that provides readout circuitry for the microphone. MEMS capacitive microphones also have a smaller dynamic range than typical piezoelectric MEMS microphones.
The noise floors of various known piezoelectric and capacitive MEMS microphones are shown in FIG. 1. As indicated by the two encircled groups of microphones, capacitive MEMS microphones (the lower group) generally have a noise floor that is about 20 dB lower than similarly sized piezoelectric MEMS microphones.
Known piezoelectric MEMS microphones have been made either as cantilevered beams or as a diaphragm, and these microphones include both electrodes and the piezoelectric material along with a structural material such as Parylene or silicon that is used as a diaphragm or beam substrate material. An advantage of Parylene for cantilever designs is that it is can be used to increase the thickness of the beam which both increases the bandwidth of the beam (for a fixed length) and increases the distance from the neutral axis of the piezoelectric material, which seemingly increases sensitivity. For example, beam substrates of about 20 μm are known, see Ledermann [15]. For piezoelectric MEMS microphones that utilize a Parylene diaphragm, thinner layers have been used. See, for example, U.S. Pat. No. 6,857,501 and Niu [10]. Note that the various references made herein to other authors are references to literature and journal articles identified at the end of this description and are provided only for non-essential subject matter in support of or as background for some of the teachings herein. Each of the referenced works are hereby incorporated by reference.
Conventional microphones additionally suffer from high noise floors due to the limited back cavity volume of the increasingly smaller microphones. As taught by the prior art, noise levels are affected by the back cavity stiffness of the microphone. More specifically, noise levels are conventionally reduced by maximizing the back cavity volume to near-infinite volumes, thereby minimizing the back cavity stiffness. While back cavity volume maximization to near-infinite volumes is feasible in larger microphone systems, this type of maximization cannot be realized in smaller microphone systems, particularly in microphones smaller than 50 mm3, where any increase in volume comes at a premium. Without the ability to minimize back cavity stiffness, conventional microphone designs either ignore the effects of the back cavity volume on microphone performance, or account for the back cavity stiffness as a parasitic stiffness.
Thus, there is a need in the piezoelectric MEMS acoustic transducer field to create a new and useful acoustic transducer with low frequency sensitivity despite residual stresses. Furthermore, there is a need in the acoustic transducer field for a new and useful acoustic transducer design that optimizes the transducer output for a given back cavity volume.